Adaptive optical system technology has proven itself invaluable to a number of applications such as astronomical imaging and long range optical communication through the atmosphere. Adaptive optical system technology can potentially enhance any application in which turbulence along the path, which leads to refractive index fluctuations due to temperature variations, degrades the performance of an imaging or laser projection system. Methods are well known in the prior art for dealing with great distances and associated phenomena of strong scintillation (wherein branch points in the phase function begin to dominate performance and amplitude fluctuations can begin to degrade performance). These methods have an important limitation that has remained unsolved throughout the history of the field: the only highly successful applications of adaptive optical systems to date are when a “cooperative” point source laser beacon at the target is provided. The point source beacon projected from the target is used to make wavefront sensing measurements of the distortions along the path for pre-compensation of a laser beam by the adaptive optical system. Many potential applications of adaptive optical systems, including laser radar, laser rangefinding, directed energy, and ophthalmic imaging all have “non-cooperative” targets. In the non-cooperative target case, no laser beacon is available from the target except that obtained from back-scattered radiation from the target itself or from the atmosphere (laser guide star obtained from Rayleigh or Mie light scattering). Many fundamental challenges exist in the case of a non-cooperative target. Overcoming these challenges would have significant benefit for many applications and open up the enabling capability for adaptive optical systems to new regimes and applications.
There is no prior art directly addressing the problem of adaptive optical systems with a non-cooperative target. There is significant prior art associated with the problem of both weak and challenging turbulence scenarios with a cooperative point source beacon. Particular recent examples include U.S. Pat. Application No. 20070176077, “System and method for correction of turbulence effects on laser or other transmission” and U.S. Pat. No. 7,113,268, “Scintillation tolerant optical field sensing system and associated method”. The former describes in detail a control algorithm for a state of the art adaptive optical system that provides optimal performance over all turbulence conditions. The latter describes an alternate means to perform wavefront sensing and correction.
The only prior art potentially addressing the topic of adaptive optical systems with a non-cooperative target is in the field of opthalmic imaging. U.S. Pat. No. 6,648,473, “High-resolution retina imaging and eye aberration diagnostics using stochastic parallel perturbation gradient descent optimization adaptive optics”. In this patent, the authors describe an imaging metric optimization based approach for correction of the effects of aberrations in the human eye. This non invasive technique offers safety advantages in that it does not require reflected back-scattered laser radiation in the human eye.
A second example that is more closely related to the present invention is in the area of optical coherence tomography. D. Merino, C. Dainty, A. Bradu, and A. G. Podoleanu, “Adaptive optics enhanced simultaneous en-face optical coherence tomography and scanning laser opthalmoscopy,” Optics Express 14(8):3345-3353 describes imaging using optical coherence tomography (a method of obtaining three-dimensional imagery of the human eye using ultra short coherence length laser light) with enhancement of the imagery obtained by using an adaptive optical system. This paper presents a method to provide improved transverse resolution by pre-compensating the ultra short coherence length laser light for aberrations in the eye. There are several critical points to be made regarding the technical approach used in this paper: (1) The wavefront sensing technique utilized was a Shack-Hartmann wavefront sensor, which works well for aberrations in the near field (i.e. near the correction plane)—these type of measurements are the integral over a subaperture of the intensity weighted gradient of the phase function of the reflected laser light; (2) For the human eye the strongest aberrations are in the near field (near the correction plane) and the approach described in this paper is expected to have good performance; and (3) because the path length difference between the reference arm and the measurement arm of the interferometer can be controlled, in the field of optical coherence tomography the interferometer can use a metrological approach and a self-referencing capability is not required.
It is known that in more demanding propagation scenarios where the aberrations are distributed along the propagation path, more advanced wavefront sensing techniques that measure the complex field directly (rather than inferred measurements of the complex field such as that obtained using a Shack-Hartmann wavefront sensor) are required. It is also known that in such scenarios the performance of conventional target return and laser guide star return based adaptive optical systems suffer significant performance degradations.
What is needed is a method for forming a near diffraction limited size beacon at a non-cooperative target. The present invention provides a solution by providing a wavefront sensing and control technique to measure the aberrations along the propagation path using return from a short coherence length laser.